The document discusses different types of transport across membranes, including: - Passive transport such as simple diffusion, facilitated diffusion, and osmosis which move substances down their concentration gradient without requiring energy. - Active transport which moves substances against their concentration gradient and requires energy from processes like the sodium-potassium pump using ATP. - Transport proteins are involved in facilitated diffusion and active transport to help move ions and polar molecules across membranes.

1. Movement across membranes

2. Concentration Gradient

3. Section 4.5
This image is a concentration gradient of black pixels, with high
concentration at the top and low concentration at the bottom.
At the top, the black pixels are
abundant and close together.
High concentration
of black pixels
Low concentration
of black pixels
At the bottom, the black pixels
are sparse.
“Gradient” Describes a Difference
Between Neighboring Regions

4. Section 4.5
The balls within this box have the same concentration gradient as
the black pixels did.
High concentration
of balls
Low concentration
of balls
This arrow points down
the concentration gradient
since it starts at high
concentration and ends at
low concentration of balls.
Concentration Gradients Have a
Tendency to Dissipate

5. Movement across membranes
• Passive Transport
– does not require input of
energy
– leads to equilibrium
– Simple diffusion
– Facilitated diffusion
– Osmosis
• Active Transport
– requires energy input
– can concentrate
substances

7. Section 4.5
In simple diffusion, particles move from high concentration to low
concentration—that is, they move down their concentration gradient.
Low concentration areas
High concentration areas
Figure 4.15
Passive Transport Does Not Require Energy

9. Section 4.5
Facilitated diffusion is passive transport that requires membrane
proteins.
Table 4.2
Passive Transport Does Not Require Energy

10. Membrane Proteins Transport Ions
and Polar Molecules
Section 4.5
The hydrophobic tails of phospholipids repel hydrophilic substances.
Therefore, ions (such as Cl- and Na+) and polar substances must pass
through a protein channel to cross the membrane.
Table 4.2
Return to AP

11. Section 4.5
Osmosis, the
diffusion of water
down its
concentration
gradient, is also a
type of passive
transport.
Figure 4.16
Passive Transport Does Not Require Energy

12. Section 4.5
Water moves toward high solute concentrations as it moves down its
concentration gradient.
Figure 4.17
Passive Transport Does Not Require Energy
All blood cells: © Dr. David M. Phillips/Visuals Unlimited

16. Turgor Pressure

17. Active Transport

18. Active Transport Requires Energy
Section 4.5
In active transport, the cell uses energy and a transport protein to move
a substance against its concentration gradient.
Table 4.2

20. For example,
the sodium-
potassium
pump uses
ATP to
transport
Na+ and K+
across the
membrane.
Figure 4.19Section 4.5
Active Transport Requires Energy

21. Endocytosis

24. Membrane Transport Summary
Is the substance
nonpolar?
Yes
No
Is the substance
moving down its
concentration
gradient?
Yes
No
Is the substance
very large?
No
Yes
Is the substance
entering or
leaving the cell?
Exocytosis
Facilitated
diffusion
Simple diffusion
Active
transport
Entering Endocytosis
Leaving
Section 4.5